If you've ever found yourself stuck in sweltering heat on a Chiltern Railways train, there's a simple explanation - they can't turn the temperature down while the train is running.

Heating settings can only be changed while the train is in a depot overnight.

The bizarre feature was revealed by a Chiltern Railways spokesperson on Twitter following a complaint by Birmingham MP Liam Byrne.

Mr Byrne, Labour MP for Hodge Hill, suffered a nightmare journey as he travelled from London to Birmingham on a Chiltern Railways train. He posted a photograph of the scene inside the carriage, complaining that passengers were "rammed in like sardines".

To their credit, a Chiltern Railways spokesperson replied to explain why the service was so packed. It appears there had been an incident affecting services by rival train operators, which meant the Chiltern train had more passengers than usual.

Mr Byrne replied: "How can we get the heat turned down? Passengers are cooking".

But the spokesperson replied: "Apologies, this can only be reset at the depot overnight. DF".

Mr Byrne responded: "You bought trains on which you can’t adjust the temperature?!"

And the Chiltern Railways official confirmed that this was correct, saying: "The train has to be taken out of service to adjust the temperature."

Mr Byrne's responded with a degree of sarcasm - and implied the company may have chosen the wrong trains. He said: "Well done to the procurement team".

So now you know. If a Chiltern Railways train is too hot, or too cold, there's nothing that can be done while the train is running.